Deficits in spontaneous and stimulus-dependent retrieval as an early sign of abnormal aging

Research on early cognitive markers of Alzheimer’s disease is primarily focused on episodic memory tests that involve deliberate retrieval. Our purpose was to provide clear evidence to support a novel Spontaneous Retrieval Deficit hypothesis, which predicts that people at pre-clinical stages of dementia, including those with amnestic Mild Cognitive Impairment (aMCI), are particularly impaired on tasks based on spontaneous retrieval. We compared 27 aMCI individuals and 27 healthy controls on mind-wandering while performing a task during which there were exposed to either highly meaningful or unmeaningful pictures. The substantial reduction in mind-wandering among individuals with aMCI was found with exposure to highly meaningful stimuli, but not to unmeaningful pictures, and it was most pronounced for past-oriented thoughts, i.e., involuntary autobiographical memories. Those findings provide strong support for this novel hypothesis, and show that it is the spontaneous, but bottom-up and cue-driven processes, for which meaningful environmental stimuli are crucial, that are very promising early markers of the disease.

www.nature.com/scientificreports/ probes. We developed two versions of the task: one with highly meaningful pictures, i.e., rated by participants as highly familiar based on their personal experience, and the other version with unmeaningful pictures, i.e., rated by participants as highly unfamiliar based on their personal experience. Strictly speaking, unmeaningful objects were not completely unknown to participants, but, judging by the familiarity ratings, they had not been present, or had been present very rarely, in participants' individual past, and therefore had no, or very little, personal meaning to them. In line with the SRD hypothesis, we expected that aMCI participants would report significantly fewer spontaneous task-unrelated thoughts than healthy controls (HC). We also expected that the reduction of spontaneous thoughts in aMCI would manifest more strongly with exposure to highly meaningful stimuli, compared to exposure to unmeaningful stimuli. Based on 28 , we also expected thoughts about past, compared to thoughts about future and present, to most strongly demonstrate the reduction of spontaneous thoughts in aMCI.

Results
The alpha level adopted for determining significance of the results was set at 0.05. The effect size was measured by partial eta squared, η p 2 (small 0.01, medium 0.06, large 0.16) or Cohen's d (small 0.20, medium 0.50, large 0.80) 38 .
Types of responses recorded during the task. To test the SRD hypothesis, and based on participants' answers whether they had any thought at a thought probe, and if yes, whether it was related to the experience of doing the Man-made/Natural Task, and whether it was spontaneous or deliberate, we grouped participants' responses into: (1) spontaneous task-unrelated thoughts; (2) spontaneous task-related thoughts (3) deliberate thoughts; (4) no thoughts. The vast majority of spontaneous task-unrelated thoughts in both groups were stimulus-dependent (91% in aMCI and 81% in HC).
To assess the hypothesis that aMCI participants would report significantly fewer spontaneous task-unrelated thoughts than HC, the mean number of thought probes in each of the 4 response types (spontaneous taskrelated, spontaneous task-unrelated, deliberate, no thoughts) were entered into a MANOVA with group (HC vs aMCI) as a between-subject factor. There was a significant main effect of group [F (3, 50) = 3.075, p = 0.036; η p 2 = 0.156] (Fig. 1). As expected, participants with aMCI experienced significantly fewer spontaneous taskunrelated thoughts than HC [F (1, 52) = 7.672, p = 0.008; η p 2 = 0.129]. There were significantly more "no thoughts" probes in aMCI individuals than in HC [F (1, 52) = 5.299, p = 0.025; η p 2 = 0.092]. No other group differences were significant (all group comparisons in "Supplementary Material").
Spontaneous task-unrelated thoughts as a function of stimulus type. To assess the hypothesis that spontaneous retrieval deficits in the aMCI group would be particularly pronounced with exposure to highly meaningful stimuli, as compared with exposure to unmeaningful stimuli, the mean number of thought probes with spontaneous task-unrelated thoughts was entered into a 2 group (HC vs. aMCI) by 2 stimulus type (highly meaningful vs. unmeaningful) mixed ANOVA with the repeated measure on the second factor. There was a significant main effect of group [F (1, 52) = 7.678, p = 0.008; η p 2 = 0.129], and a significant group by stimulus type interaction [F (1, 52) = 9.728, p = 0.003; η p 2 = 0.158] (Fig. 2). As predicted, aMCI participants reported significantly fewer spontaneous task-unrelated thoughts than HC when exposed to highly meaningful stimuli [F (1,52)    www.nature.com/scientificreports/ Past-oriented thoughts (involuntary autobiographical memories) as a function of stimulus type. Since the reduction of spontaneous task-unrelated thoughts in aMCI was most strongly pronounced in involuntary memories, we conducted additional analyses to investigate whether the quality of stimuli influenced the size of the reduction in memories in the same way as it was predicted, and indeed found, for spontaneous task-unrelated thoughts overall. Therefore, the mean number of thought probes with past-oriented, spontaneous and task-unrelated thoughts was entered into a 2 group (HC vs. aMCI) by 2 stimulus type (highly meaningful vs. unmeaningful) mixed ANOVA with the repeated measure on the second factor. There was a significant main effect of group [F (1, 52) = 21.482, p = 0.000; η p 2 = 0.292], and a significant group by stimulus type interaction [F (1, 52) = 5.348, p = 0.025; η p 2 = 0.093] (Fig. 4). Individuals with aMCI had significantly fewer past-oriented thoughts than HC for both highly meaningful and unmeaningful stimuli, but, as it could be expected, the effect size of this difference was much bigger for highly meaningful stimuli Potential confounds to spontaneous retrieval deficits. It may be argued that the Man-made/Natural Task was easier, and therefore more boring, for HC, compared to aMCI individuals, which made them mindwander more. In a similar vein, the Man-made/Natural Task may have been more difficult for aMCI individuals, and therefore they did not have enough cognitive resources left for spontaneous processes. However, the data indicate otherwise (Table 1). Both groups performed at ceiling on the Man-made/Natural Task, and they did not differ on any of the performance measures (the percentage of correct answers out of all answers provided and mean response time), except for the number of invalid answers caused by pressing the wrong keyboard button or missing an answer. Furthermore, the groups expressed the same level of interest in the task. However, to exclude this alternative explanation of group differences in mind-wandering, we investigated whether the level of performance on the Man-made/Natural Task influenced the pattern of group differences in the number of . Mean number of thought probes with spontaneous, past-oriented, and task-unrelated thoughts as a function of stimulus type (highly meaningful vs unmeaningful) and group (aMCI participants vs. healthy controls). Table 1. Mean (standard deviation) accuracy, response time, invalid answers and interest ratings for the manmade/natural task in participants with aMCI and healthy controls, and results of independent samples T-test. aMCI amnestic Mild Cognitive Impairment. Differences between aMCI and HC are indicated by ** p < 0.01. a Except for the interest ratings that were not provided by two participants (one in each group). b Task interest ratings were made on a 10-point scale (1 = very boring; 10 = very interesting). www.nature.com/scientificreports/ spontaneous task-unrelated thoughts. The mean number of spontaneous task-unrelated thoughts was entered into a one-way ANCOVA, with group as a between-subject factor and the three measures of performance on the Man-made/Natural Task  , such that aMCI participants continued to mind-wander less, after controlling for performance on the Man-made/Natural Task. This speaks against the task difficulty as being a potential confound to spontaneous retrieval deficits.

Man-made/natural task aMCI (n = 27) a Healthy controls (n = 27) t df p d
It may also be argued that the group differences in cognitive functions, other than memory retrieval, may explain less mind-wandering in aMCI. Again, to exclude this alternative explanation, we investigated whether other cognitive functions, as measured by the Addenbrooke's Cognitive Examination-III (ACE-III), influenced the pattern of group differences in the number of spontaneous task-unrelated thoughts. The mean number of spontaneous task-unrelated thoughts was entered into a one-way ANCOVA, with group as a between-subject factor and a composite score on the ACE-III, which included the attention, fluency, language and visuospatial abilities subscales, as a covariate. The covariate was not significant [F (1, 51) = 0.126 p = 0.724; η p 2 = 0.002]. The main effect of group was significant [F (1, 51) = 6.282; p = 0.015; η p 2 = 0.110], such that aMCI participants continued to mind-wander less, after controlling for performance on the ACE-III. This speaks against the differences in other cognitive functions as being potential confounds to spontaneous retrieval deficits.
The aMCI group and HC did not differ in their ratings of how difficult the task of categorizing thoughts was for them (p = 0.265).

Discussion
A recently formulated SRD hypothesis stipulates that tasks based on spontaneous retrieval are most compromised in aMCI and at early stages of AD, and are better early cognitive markers of the disease, compared to tasks that rely on deliberate episodic memory processes 9 . This hypothesis is highly counterintuitive because it challenges current theories of cognitive aging 39,40 which predict that both typical and atypical aging predominantly impair performance on difficult cognitive tasks that rely on deliberate and strategic processes. It also speaks against the current practice of the dementia diagnosis which involves neuropsychological tests based on strategic encoding and retrieval processes. However, recent neuropsychological studies have shown that the structures responsible for spontaneous retrieval degenerate much earlier during the dementia development than those mediating strategic memory processes 10,11,[14][15][16] .
The primary goal of the present study was to provide more conclusive behavioral evidence to support the SRD hypothesis. To this aim, we compared individuals with aMCI and healthy controls in terms of mind-wandering while performing the task that met all the criteria to capture spontaneous stimulus-dependent retrieval, and included either highly meaningful or unmeaningful pictorial stimuli. Several important findings emerged from this comparison that provide very strong support for the SRD hypothesis.
Most importantly, in line with the SRD hypothesis, individuals with aMCI experienced significantly less spontaneous task-unrelated thoughts than HC. Second, we demonstrated the robustness of the spontaneous retrieval deficits by showing that, for the first time, for pictorial material. Third, the present study is the first to demonstrate that the quality of stimuli in the environment, which could potentially trigger spontaneous thoughts, impacts the size of the spontaneous retrieval deficits. This finding unequivocally supports the claim that the deficits involve spontaneous, but the bottom-up and cue-driven, processes. As predicted, the reduction in spontaneous task-unrelated thoughts was found with exposure to highly meaningful stimuli, but not to unmeaningful pictures. Finally, in accordance with 28 and our hypothesis, the deficits were most pronounced for past-oriented, spontaneous, task-unrelated thoughts (involuntary autobiographical memories). For such thoughts the deficits were significant for both highly meaningful and unmeaningful stimuli, but varied in size: they were much larger for highly meaningful pictures.
Unexpectedly, we found an increase in the number of spontaneous task-unrelated thoughts in aMCI individuals for unmeaningful stimuli, as compared with highly meaningful pictures. When only past-oriented, spontaneous, task-unrelated thoughts were taken into account, mind-wandering no longer significantly differed after the exposure of aMCI participants to unmeaningful stimuli versus highly meaningful stimuli. This suggests that present-oriented thoughts may have been primarily responsible for this unexpected increase (future-oriented thoughts were scarce). Such interpretation is supported by the fact that the type of stimuli did not influence the number of present-oriented, spontaneous, task-unrelated thoughts, either in the between-groups or withingroups comparisons. This explanation is also in line with the studies showing that, in the absence of meaningful stimuli, people tend to experience primarily future-and present-oriented thoughts 41,42 in which the deficits are less pronounced compared to involuntary autobiographical memories (see 28 and the present findings). Interestingly, although future-oriented thoughts were much less frequent than present-oriented thoughts in the present study, it is future-oriented thoughts that demonstrated the reduction of mind-wandering in aMCI, albeit a much smaller reduction than that for autobiographical memories. This finding is in line with the results of both neuroimaging research [43][44][45] and behavioral studies [46][47][48][49] showing that past-oriented and future-oriented thoughts are based on overlapping cognitive processes. These studies suggest that future-oriented and past-oriented thoughts can be considered two aspects of the same phenomenon, i.e., mental time travel which is the ability to mentally re-experience autobiographical events and pre-experience possible future occurrences 50 .
A possible limitation of our study was using captioned pictures in the Man-made/Natural Task, rather than pictures alone. Although thought probes explicitly asked participants whether they had any picture-related thoughts, and no participant mentioned captions, participants might have difficulties in distinguishing between caption-induced thoughts and picture-induced thinking. This limitation does not change the fact that we www.nature.com/scientificreports/ extended behavioral data in support of reduced mind-wandering in aMCI to the type of stimuli that had not been used in previous supportive studies. However, it may lead to a slightly different theoretical interpretation of this reduction. If thoughts were caption-induced, then semantic-to-autobiographical memory priming may have been involved 51,52 . This priming takes place when processing semantic information (prime) leads to activation of relevant autobiographical knowledge structures which increases the likelihood of evoking related memories. Mace et al. 51 consider semantic-to autobiographical priming a specific type of associative priming, occurring between two separate memory systems (semantic and autobiographical). It has been demonstrated in relation to involuntary autobiographical memories, and interestingly, all primed memories were associated only with highfrequency prime words 51,52 . Mace et al. 51,52 suggest that low-frequency prime words are weakly associated with participants' personal experience, and therefore may activate very few autobiographical memories. It is likely that, in the present study, caption words for unmeaningful objects were lower in frequency, compared to captions for highly meaningful objects. Therefore, it may be argued that the reduced number of involuntary autobiographical memories in the MCI group was due to impaired spreading of activation between semantic representations of verbal primes and related autobiographical memories. It may be further argued that low-frequency caption words, corresponding to unmeaningful objects, were less able to demonstrate this impairment because they had equally poor associations with participants' personal experience in both aMCI group and healthy older adults. Future research may test this theoretical interpretation of differences between aMCI and healthy ageing. However, it should be noted that even this alternative interpretation puts emphasis on those deficits in aMCI that are related to automatic/spontaneous processes in memory.
As for practice, our findings may help researchers to develop new and simple cognitive tests to assess spontaneous, stimulus-driven processes, which may be used clinically for detecting early cognitive deterioration and predicting the conversion to AD. In addition to meeting the criteria listed in the Introduction, e.g., undemanding ongoing task, thought probes, distinguishing between spontaneous and deliberate thoughts, such tests should provide patients with highly meaningful environmental stimuli. Our findings suggest that, for healthy older adults, in contrast to individuals with aMCI, such an environment stimulates spontaneous task-unrelated thoughts in general, and past-related thoughts in particular.

Method
Participants. A total of 27 healthy older adults and 27 aMCI participants were recruited. To ensure sufficient power, we performed the a priori power analysis on GPOWER 3.1 53 . The effect size calculation was based on mind-wandering reported by Niedźwieńska and Kvavilashvili 28 (f = 0.718). With an alpha level of 0.05 and the minimum power of 0.95, 28 participants were necessary to find a statistically significant effect in the model. However, in the study of Maillet and Schacter 6 , in which the Man-made/Natural Task was originally used to compare young adults with healthy older adults, older adults reported much more "no thoughts" trials (~ 20%), compared to the task used in the reference study of Niedźwieńska and Kvavilashvili (6%) 28 . This suggested that the Maillet and Schacter task might have been less powerful in eliciting mind-wandering. Although the substantially modified version of the Maillet and Schacter task was used in the present study, to avoid the risk of not having enough power to capture the difference in mind-wandering between aMCI individuals and HC, we recruited about twice as many participants as calculations indicated were necessary.
All participants were recruited from among inhabitants of local nursery houses and members of senior social clubs. All research was performed in accordance with the Declaration of Helsinki. The study was approved by Psychology Research Ethics Committee at the Jagiellonian University. Participants provided written informed consent to take part in the study. For all participants, exclusion criteria included: (a) head/brain injuries, (b) history of cerebrovascular disease, (c) current alcohol or substance dependence, (d) medical, neurological, or psychiatric disorders resulting in cognitive dysfunctions, (e) age less than 65 years. Fluency in Polish and adequate vision and hearing were also required. Exclusion criteria were assessed in the initial interview screening. Participants who passed the screening, completed a battery of experimental and standardized neuropsychological tests. Healthy controls (HC). Inclusion criteria for the HC group were: (a) a score within or above 1.5 SD of the mean of age-matched peers on each test of the neuropsychological screening battery assessing episodic memory; (c) a score ≥ 27 on the MMSE; (d) no impairment in instrumental activities of daily living, as confirmed by minimum score in the Instrumental Activities of Daily Living (IADL) subscale of Nurses' Observation Scale for Geriatric Patients (NOSGER) 58,59 ; (e) absence of severe depression, as confirmed by a score of below 10 on the GDS 15. Table 2 shows demographic details of the final sample. A series of independent samples t-tests revealed no significant group differences between aMCI and HC on the demographic variables, except for MMSE scores, which were lower in aMCI individuals than in HC (p = 0.000; d = 1.74).  65 . Significant group differences were obtained for all neuropsychological tests, with aMCI participants scoring lower (see Table 3). The effect sizes for episodic memory tests were markedly higher than for the tests measuring other cognitive functions i.e. attention, language, fluency and visuospatial.
Mind-wandering evaluation. Participants completed a computer-based Man-made/Natural Task, which was a modified version of the task developed by Maillet and Schacter 6 . The task consisted of a 242-slide presentation of pictures showing natural objects (e.g., flower) and man-made objects (e.g., car). Below each picture there was a caption corresponding to it. Participants were asked to decide whether the depicted object was artificial or natural. Each stimulus was presented for 4 s, followed by a blank screen for 4 s. Every 6-10 stimulus slides, the task stopped and thought probe questions appeared on the screen. Participants were asked to describe their thought content the moment before the question appeared on the screen by choosing one of the following answers: (a) I did not have any thoughts; (b) I had a thought triggered by one of the pictures I saw; (c) I had a thought unrelated to the task or any of the pictures I saw; (d) I was thinking how I feel about doing this task. If participants had stimuli-related thoughts, they were additionally asked which picture had triggered the thought. Participants were then asked whether the thought they had was spontaneous or deliberate. Finally, they were asked whether the thought they had were about the past, present or the future. The categories of thoughts and the thought probing procedure were adapted from Maillet and Schacter 6 (see also 28,32 for similar thought probing). Thought  www.nature.com/scientificreports/ probes were presented 1.5 s after preceding stimulus slides, since the results of Maillet and Schacter 6 suggest that such interval slightly increases the probability of evoking stimulus-related thoughts in healthy adults. Stimulus presentation and the response collection were controlled by Inquisit 5 software running on a 14″ foldable notebook. Pictures measured on average 600 px (height) × 600 px (width) at a viewing distance of 60 cm, and were presented on a white background in the center of the screen. They were generated in a random order, which was then the same for each participant. To simplify the recording of thought probes for Polish older adults who may not be very familiar with using the computer, all participants were giving their answers orally, rather than typing them into the computer as in Maillet and Schacter 6 . The experimenter manually recorded participants' responses.
We developed two versions of the Man-made/Natural Task. All participants completed the two versions in one session, in a counter-balanced order, without any break between the versions. Each version consisted of 121 blocked pictures of either very familiar objects (the block with highly meaningful stimuli) or unfamiliar objects (the block with unmeaningful stimuli). There were 15 thought probes in each block.
Stimuli-pictures were obtained from the same base as used by Maillet and Schacter 6 , i.e., Bank of Standardized Stimuli 66,67 . The base consists of stimuli that were assessed on different dimensions by a high number of participants, as part of normalization studies. One of these dimensions was familiarity, which was measured with the question: "Rate the level to which you are familiar with the object" on a 5-point scale (1 = very unfamiliar; 5 = very familiar).
For the present study, 300 pictures with the highest scores of familiarity and 300 pictures with the lowest scores of familiarity were chosen from the base. To select the pictures that would be most familiar/unfamiliar to Polish older adults, the pilot study was conducted in which 29 Polish older adults of age 60 + ( At the beginning of Session 2 participants completed the short-delay HVLT tasks. They were then briefly introduced to the Man-made/Natural Task. Participants were asked to press "S" on the keyboard if the object on the screen was man-made, and press "N" if it was natural. They were also informed that we are interested in what types of thoughts people experience while performing such tasks. Therefore, the slide presentation would occasionally stop, at which point they would be prompted to report their thoughts at the exact moment they were stopped. Participants were briefly informed about the thoughts they might experience during the task and what options they would have to categorize them, i.e., no thoughts, picture-triggered off-task thoughts, pictureunrelated off-task thoughts, and thoughts about the experience of performing the task. This was followed by training, during which participants were given examples of thoughts from various categories and asked what category they would choose. If they made the wrong choice, they were explained why it should be a different category. The exemplary thoughts were, among others: I used to work as a bus-driver after seeing the picture of a bus; I have a doctor appointment tomorrow, with a no picture related to such fact; I wonder if I have chosen the right answer. The training continued until the participant was able to correctly categorize all types of thoughts. Participants were then explained the difference between spontaneous thoughts (thoughts that pop into mind without your intention) and deliberate thoughts (something you deliberately chose to think about). Finally, participants were briefly informed about the types of off-task thinking they could experience, i.e., that it could be related to something that: (a) was happening in the present, at any point in the course of the task (e.g., I love my family); (b) had happened in the past, before starting the task (e.g., I went to Spain last year); (c) would happen in the future, after completing the task (e.g., I'm going to eat delicious supper today). This was followed by short practice with two 10-slides trials and two thought probes. After practice, participants completed the long-delay HVLT tasks and then both versions of the Man-made/Natural Task. When the procedure was completed, participants were asked to rate how interesting the task of classifying pictures was (1 = very boring; 10 = very interesting), and how difficult the task of categorizing thoughts was (1 = very difficult; 10 = very easy).

Data availability
The data used to support the findings of this study are available from the corresponding author upon request. www.nature.com/scientificreports/